You can get a heck of a lot of computer for that sort of money. Check out HP and dell, maybe alienware, though that will be pricey. Falcon Northwest and ABS also are good performance. I'd definitely consider building your own as well, since that would most likely save you some money. Just comparison shop and see how much you can get for your money.

While AMD Athlon 64 is the way to go, it is already known that the current socket will be replaced shortly with Socket 939, rendering current hardware basically obsolete. This doesn't mean it won't work, only that in the near future current Socket 754 will no longer be available. That's why I'd wait until the platform matures a bit more...